The state of the art can be defined by the following documents.
The document WO-A-89/09895 has for its object an apparatus with gearing which comprises a first gear element, as well as a second and a third gear element engaging with said first gear element, such that the rotation of the first gear element is transmitted to the second and third gear elements, or such that the rotation of the second and third gear elements is transmitted to said first gear element. Said first gear element comprises assemblies of elongated gear teeth defining a portion of a conical shape, the gear teeth, located in each assembly, being parallel to each other, and several of said gear teeth in each assembly being of different lengths. The second and third gear elements are connected such that they turn at the same speed during operation of said gear apparatus. Said second and third gear elements are movable along the gear teeth of each assembly, so as to vary the gear ratio between the first gear element and the second and third gear elements.
Although of novel structure, this apparatus is complex in structure, with a very large number of moving parts. The use of a central cone, as a first gear element, is consumptive of space. This volume is greater as the number of ratios is greater. Finally, between the tooth assemblies, there are smooth regions in which the engagement of the second and third gear elements is not carried out; there is thus a differential force on the teeth of these elements, which is not at all adequate for reliability of the apparatus.
The document FR-A-2.640.342 discloses a speed varier, with gearing always engaged, which is provided with a shaft (and its pinion, one or the other) sliding parallel to the principal conical gearing, and by a cone constituted of stacked toothed wheels which are not joined and by two toothed spirals, one rising, the other descending, which cross without interference of the teeth of the intervals thanks to their number and specific dimensions and to their male shape (pinion) and female shape (cone) such that the pinion engages, either in a circular trajectory (speed ratios) without axial pressure on the shaft, or on the spirals (with axial pressure), but not between the two. The figure represents the projection on a plane of the principal conical gear, at the level of the diameter of the foot. The beveled lozenges represent the impression of the tips of the teeth of the pinion; the double concave bevel teeth are taken en masse.
This device, like the preceding one, uses a cone constituted by stacked toothed wheels, such that it is voluminous with many moving parts.
The document FR-A-877.542 provides a gear system having teeth constituted by one or several rows of movable balls disposed in recesses provided on primary diameters of independent bodies, engaging in corresponding recesses or roller tracks provided in the primary diameters of the wheels, pinions, or plates with which they are to be coupled.
The gear system proposed here requires the use of heavy and costly equipment because the plate must be releasable and the pinion must be manipulated. There are thus necessarily two independent manipulating mechanisms.
The document GB-A-2.263.953 relates to a gear system which can consist in one or two hard metal disks in which have been cut recesses of a shape suitable for the intervals between chords (all equal in length), to form circles of suitable selected radii, in which a movable toothed wheel or pinion can be engaged by means of a selection mechanism with radial sliding when the disk is in stationary position at the level of certain points of alignment, called primary radii, thereby permitting a speed selection and coupling loads to drive it. A combination of gear disks is provided with a single conductive shaft, for rotation in a drill or the propulsion of a bicycle, which ensures a wide range of speed change (relative speed and torque), and the movement of rotation will be transmitted from the disk having the recesses, which turn under the action of pedals, to a toothed wheel secured to a squared or keyed shaft which transmits the movement to a similar toothed wheel, located at the other end of the shaft, which is in engagement, during its rotation, with the disk having recesses, secured to the rear wheel, which propels the bicycle.
In this case, clutching is ensured by the displacement of the pinion relative to the plate at the level of the first radii, which are in fact substantially radial grooves in the plate. There are three essential drawbacks in this system. In the first place, there is wear of the teeth of the pinion because of the translation of the level of the first radii. Secondly, the time available to carry out the speed change is limited to the time of engagement of the two adjacent teeth of the pinion. Thirdly, the solidity and balance of the plate, which have two grooves at a precise position, are not guaranteed.
The present invention provides simple and low volume mechanism which solves the problems existing in the prior art.